The mechanical and metabolic energetics of locomotion are ultimately determined by the mechanical properties of skeletal muscles and the pattern of contraction they undergo. The link between muscle properties and movement energetics is poorly developed because we lack information about how muscles contract in vivo. The proposed research will use a particularly suitable locomotor model to measure force, power and activity of muscles and tendons directly during running, walking and acceleration. Direct measurements of muscle contraction in vivo, measurements of tissue properties, and inverse dynamics will be used to determine how muscle contractile power is translated into movement. Independent measurements of muscle and tendon work will be used to test the hypothesis that tendon energy recovery supplies the majority of the positive work of movement during steady-speed level walking and running. The force-velocity and length-tension properties of muscles will be used to test the hypothesis that muscles operate at lengths and shortening velocities that allow for economical force production during steady-speed walking and running. The role of passive force development during movement will also be investigated to test the hypotheses that muscles produce force passively over a range of muscle lengths and velocities where active force capacity is low. These studies will provide insight into how the cellular and molecular properties of muscles and tendons determine the energetics and mechanics of normal gait. A basic understanding of muscle mechanical function during normal gait is important for developing rehabilitative therapies for individuals with musculoskeletal injuries or gait disorders, the design of prosthetic devices, and an understanding of the mechanical forces that influence the regulation of muscle properties.